In the completion of oil and gas wells, downhole tools are mounted on a workstring, such as a drill string, a landing string, a completion string, or a production string. The workstring can be any type of wellbore tubular, such as casing, liner, tubing, and the like. A common operation performed downhole temporarily obstructs the flow path within the wellbore to allow the internal pressure within a section of the workstring to be increased. In turn, the increased pressure operates hydraulically actuated tools. For example, a liner hanger can be hydraulically operated to hang a liner in the well's casing.
Sealably landing a ball on a ball seat provides a common way to temporarily block the flow path through the wellbore tubular so a hydraulic tool above the seat can be operated by an increase in pressure. Historically, segmented dogs or keys have been used create the ball seat for landing the ball. Segmented ball seats may be prone to fluid leakage and tend to require high pump rates to shear open the ball seat. Additionally, the segmented ball seat does not typically open to the full inner diameter of the downhole tubular so the ball seat may eventually need to be milled out with a milling operation.
Alternatively, a hydro-trip mechanism can use collet fingers that deflect and create a ball seat for engaging the dropped ball. In this type of ball seat, the collet-style mechanism opens up in a radial direction when shifted past a larger diameter groove. However, the collet-style ball seat is more prone to leaking than solid ball seats, and the open collet fingers exposed inside the tubular create the potential for damaging equipment used in subsequent wellbore operations.
Any of the hydraulic tools that are to be actuated and are located above the ball seat need to operate at a pressure below whatever pressure is needed to eventually open or release the ball seat. Internal pressures can become quite high when breaking circulation or circulating a liner through a tight section. To avoid premature operation of the tool at these times, the pressure required to open or to release a ball seat needs to be high enough to allow for a sufficiently high activation pressure for the tool. For example, ball seats can be assembled to open or release at a predetermined pressure that can exceed 3000 psi.
Once the hydraulically-actuated tool, such as a liner hanger or packer is actuated, operators want to remove the obstruction in the tubular's flow path. Since the ball seat is a restriction in the wellbore, it must be opened up, moved out of the way, or located low enough in the well to not interfere with subsequent operations. For example, operators will want to move the ball and seat out of the way. Various ways can be used to reopen the tubular to fluid flow.
Commonly, the ball seat is moved out of the way by having it drop downhole. For example, with the ball landed on the seat, the increasing pressure above the ball seat can eventually cause a shearable member holding the ball seat to shear, releasing the ball seat to move downhole with the ball. However, this leaves the ball and ball seat in the wellbore, potentially causing problems for subsequent operations. Additionally, this may require the removal of both the ball and ball seat at a later time.
In another way to reopen fluid flow through the tubular, increased pressure above the ball seat can eventually force the ball to deformably open the seat, which then allows the ball to pass through. In these designs, the outer diameter of the ball represents a maximum size of the opening that can be created through the ball seat. This potentially limits the size of subsequent equipment that can pass freely through the ball seat and further downhole without the risk of damage or obstruction.
Ball seats may also be milled out of the tubular to reopen the flow path. For example, ball seats made of soft metals, such as aluminum or cast iron, are easier to mill out; however, they may not properly seat the ball due to erosion caused by high volumes of drilling mud being pumped through the reduced diameter of the ball seat. Also, if additional landings are to be made, interference from the first ball seat being released downhole may also prevent the ball from sealably landing on another ball seat below.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.